1. Field of Invention
Exemplary aspects of the present invention relate to a method and apparatus for image forming, and more particularly, to a method and apparatus for image forming capable of effectively correcting an output from a toner density sensor.
2. Description of the Related Art
A related art image forming apparatus has employed a two-component development method commonly known in the art. This two-component development method develops an image by carrying a two-component developer (hereafter referred to as a developer) including a non-magnetic toner and a magnetic carrier on a development sleeve as a developer carrier, forming the developer in a magnetic-brush-like shape on the development sleeve by an action of magnetic poles included in the development sleeve, and applying a development bias to the development sleeve at a location opposing to a photoconductor as a latent image carrier. This two-component development method is advantageous in color image forming and, consequently, has been widely employed. In the two-component development method, the developer is carried to a development region with a rotation of the development sleeve. According to this movement of the developer, a large amount of the magnetic carriers in the developer are gathered with attached toner particles along lines of magnetic force of the development poles so that the developer is formed in a magnetic-brush-like shape.
Unlike a one-component development method, the two-component development method is believed to be important to efficiently control a weight ratio (referred to as a toner density) between a toner and the carrier to enhance stability. For example, when the toner density is excessively high, a background soiling is generated on the image, and a detail resolving power is decreased. When the toner density is low, deterioration of a solid image density or adhesion of the carrier is generated. Thereby, the toner quantity supplied to the developer is controlled, and the toner density in the developer needs to be controlled within an appropriate range. The toner density is controlled by comparing an output value Vt of a permeability sensor, serving as a toner density detection mechanism, with a reference value Vref density, and arranging the toner supply quantity based on a result of the comparison.
The permeability sensor is generally used to detect the toner density as permeability. The sensor detects a permeability variation of the developer caused by a variation of the toner density of the developer, and compares the output of the sensor with the reference density so as to determine the current value of the toner density. Another method uses an optical sensor toner density. The result detected by the optical sensor detects a reflection density of an image area and a non-image area of a reference pattern, which is formed on an image carrier or an intermediate transfer belt, so as to determine the toner density.
Another publicly known method is to control the reference value Vref of the permeability sensor based on a detection result of a toner adhesion amount of the reference pattern, which is formed between each of image outputs (between sheets), even during image forming operation. However, when the reference pattern is formed between the sheets, the toner is excessively consumed. This excess consumption of the toner needs to be reduced. Thereby, there is a tendency not to control the Vref by forming the reference pattern between the sheets. When the reference pattern is formed on the intermediate transfer belt, a cleaning device needs to be disposed on a secondary transfer roller. Thereby, there is a tendency not to form the reference pattern between the sheets from a cost reduction point of view. In such a case, the toner density needs to be correctly controlled by the permeability sensor solely when the images are continuously formed or an image mode is changed, such as the process linear velocity.
One example has attempted to detect the toner density of the developer in a development device by using the permeability sensor as the toner density detection mechanism, comparing a result detected by the permeability sensor with a threshold value, controlling the toner density in the development device based on a result of the comparison, and changing the threshold value with respect to a detection value of the toner density detection mechanism in response to a variation of a photoconductor linear velocity.